LMH6517 Datasheet, PDF (25/34 Pages) National Semiconductor (TI) – Multi Standard, IF and Baseband, Dual, DVGA

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LMH6517
SNOSB19K – NOVEMBER 2008 – REVISED MARCH 2013
UP/DN
PULSE TIMING
t_PG
t_PW
RESET TIMING
UP
t_RW
DN
Figure 56. Pulse Mode Timing
EXPOSED PAD WQFN PACKAGE
The LMH6517 is packaged in a thermally enhanced package. The exposed pad is connected to the GND pins. It
is recommended, but not necessary, that the exposed pad be connected to the supply ground plane. In any
case, the thermal dissipation of the device is largely dependent on the attachment of this pad. The exposed pad
should be attached to as much copper on the circuit board as possible, preferably external copper. However, it is
also very important to maintain good high speed layout practices when designing a system board. Please refer to
the LMH6517 evaluation board for suggested layout techniques.
Package information is available on the Texas Instruments web site.
http://www.ti.com/packaging/
INTERFACING TO ADC
The LMH6517 was designed to be used with high speed ADCs such as the ADC16DV160. As shown in
Figure 40, AC coupling provides the best flexibility especially for IF sub-sampling applications. For DC coupled
applications the use of a level shifting amplifier or a resistive biasing network may be possible.
The inputs of the LMH6517 will self bias to the optimum voltage for normal operation. The internal bias voltage
for the inputs is approximately mid rail which is 2.5V with the typical 5V power supply condition. In most
applications the LMH6517 input will need to be AC coupled.
The output common mode voltage is also self biasing to mid supply. This means that for driving most ADCs AC
coupling is required. Since most often a band pass filter is desired between the amplifier and ADC the bandpass
filter can be configured to block the DC voltage of the amplifier output from the ADC input.
100:
390 nH ^{3 pF}
100:
390 nH
41 pF
3 pF
27 nH 200:
200
Figure 57. Bandpass Filter
A. Center Frequency is 140 MHz with a 20 MHz Bandwidth. Designed for 200Ω Impedance
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